Presses used in compression molding have generally comprised a rigid steel frame with a base, a pair of opposing side slabs and an upper cross-head. An upper platen is fitted between the side slabs and guided on appropriate guide surfaces machined or otherwise provided on the side slabs. A lower platen, positioned to confront the upper platen, is supported from the base by a multiplicity of short-stroke hydraulic cylinders. A large bore, long-stroke, central ram, suspended from the cross-head, has generally served as means for advancing the upper platen towards the lower platen, to reduce the substantial separation existing between an upper mold member and a lower mold member, commonly fixed to the platens at the start of the molding cycle. The upper platen is commonly advanced until it engages mechanical stops attached to the side slabs, with a comparatively small separation between the mold members. The ram is then allowed to continue operating under full working-pressure against the mechanical stops during the subsequent compression stage of operation. The lower platen is then be advanced by means of the four short-stroke hydraulic cylinders to compress the upper and lower mold members, and more specifically, to compress a charge of molding compound placed between the mold members. The upward force of the short-stroke cylinders is of course resisted by the central cylinder suspended from the cross-head.
Sensors are positioned at a number of locations between the upper and lower platens to provide information regarding relative tilting of the platens. Appropriate controls responsive to the sensors regulate operation of the four short-stroke cylinders, individually, to ensure parallel pressing. Compression molding processes operating in this manner are well known, and a typical parallelism control is described, for example, in U.S. Pat. No. 4,076,780 which issued on Feb. 28, 1978 to Edwin D. Ditto.
Such presses have a number of disadvantages. The central ram, which must have both a long-stroke and a capacity to accomodate the full compressing force generated by the press, is large and expensive. Cycle time tends to be very slow, because of the time delay inherent in advancing a large bore, large-stroke cylinder. A large reservoir of hydraulic fluid is required to accomodate the capacity of the central cylinder and the expansion required of the cylinder. The housing must be massive and very robust to withstand the forces generated during compression.
Presses which embody a high degree of parallelism control and which eliminate the need for a large central ram and robust support structure have recently become available. Such presses incorporate a moveable lower platen supported by a multiplicity of short-stroke, large bore hydraulic cylinders which are actuated only during the compression phase of operation. A moveable upper platen is interfitted with cylindrical guide posts, rigidly fixed to an associated support structure, that guide the upper platen towards and away from the lower platen. The separation between the platens can be quickly reduced by means of small bore, long-stroke cylinders which act between the frame and upper platen until the compression phase of operation can be commenced. What is believed to be a hydraulically-actuated, chuck-like clamping mechanism, carried in the interior of and fixed to the upper platen, is then actuated to friction grip the cylindrical guiding surfaces of the posts thereby fixing the upper platen to the guide posts. The lower platen is then advanced by means of the short-stroke cylinders, with sensors regulating operation of the cylinders to ensure parallelism, to compress a work-piece between the platens. Such presses are very expensive, and, because the lower platen must move, lack versatility, not readily permitting the implementation, for example, of certain high-pressure injection molding processes in which mold members are clamped between upper and lower press platens and a nozzle engaged with the mold.
A press design is disclosed which lends itself to parallelism control, which is comparatively inexpensive and which has a stationary lower platen.